Rotor assembly for an exhaust gas turbocharger

ABSTRACT

In a rotor assembly for an exhaust gas turbocharger including a turbine wheel and a compressor wheel mounted on a common shaft for joint rotation wherein the turbine wheel consist of a metal aluminide or of a high-temperature resistant titanium alloy, the turbine wheel and the compressor wheel are disposed on the shaft in spaced relationship by way of a bearing sleeve via which the turbien wheel and the compressor wheel are axially firmly engaged by axial clamping structures associated with the common shaft.

This is a Continuation-In-Part Application of pending Internationalpatent application PCT/EP2008/001996 filed Mar. 13, 2008 and claimingthe priority of German patent application 10 2007 012 641.9 filed Mar.10, 2007.

BACKGROUND OF THE INVENTION

The invention relates to a rotor assembly for an exhaust gasturbocharger including a compressor wheel and a turbine wheel mounted ona common shaft, wherein the turbine wheel consists of a metal aluminideor a high temperature-resistant titanium alloy.

DE 10 2005 015 947 B3 discloses a method for the connection of a firstcomponent of a metal aluminide or a high-melting Ti alloy with a secondcomponent of steel, wherein the connection is produced by a frictionwelding process. The two components can be joined in a positive mannerby means of a nickel-containing intermediate piece. The presence of twojoints is characteristic for the connection. This method servesespecially for the production of a rotor assembly of an exhaust gasturbocharger, which comprises a turbine wheel consisting of aluminide orof a high-melting Ti alloy and a shaft of steel.

The advantage of a turbine wheel of a metal aluminide or a high-meltingTi alloy resides in a lower weight and, consequently, a reduction of themoment of inertia of the turbine wheel, whereby the response-behavior ofan exhaust gas turbocharger is considerably to be improved.

It is the object of the present invention to provide a rotor assemblywhich comprises a reliable connection between a turbine wheel of a metalaluminide or a high-melting Ti alloy and a shaft of steel even at hightemperature and high rotational speeds of the rotor assembly.

SUMMARY OF THE INVENTION

In a rotor assembly for an exhaust gas turbocharger including a turbinewheel and a compressor wheel mounted on a common shaft for jointrotation wherein the turbine wheel consist of a metal aluminide or of ahigh-temperature resistant titanium alloy, the turbine wheel and thecompressor wheel are disposed on the shaft in spaced relationship by wayof a bearing sleeve via which the turbien wheel and the compressor wheelare axially firmly engaged by axial clamping structures associated withthe common shaft.

Due to the rotationally fixed connection between the turbine wheel andthe compressor wheel via the shaft of the rotor assembly and a shaftsleeve disposed between the turbine wheel and the compressor wheel, anexhaust gas turbocharger having a reduced moment of inertia and thus animproved response behavior with high operational safety can be realized.

In one arrangement, the rotationally fixed connection can be made bymeans of at least one tensioning element arranged at one end of theshaft. A positive connection is thereby either provided between theother end of the shaft and the turbine wheel, or the other end comprisesa further tensioning element for providing the axial engagement.Alternatively, the other end itself may be used to form a tensioningelement. A secure positive connection between the compressor wheel andthe turbine wheel can thus advantageously established by axialengagement which is not affected by the centrifugal forces and the hightemperatures to which the turbine wheel is objected during high speedoperation of the turbocharger.

The invention will become more readily apparent from the followingdescription thereof on the basis of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows, in a longitudinal sectional view, a first embodiment of arotor assembly according to the invention,

FIG. 2 shows a second embodiment of the rotor assembly wherein theturbine wheel of the rotor assembly comprises an integral elongatedcollar,

FIG. 3 shows a third embodiment of the rotor assembly in a thirdversion, wherein the turbine wheel comprises a positive connection witha bearing sleeve,

FIG. 4 shows a fourth embodiment of the rotor assembly, wherein theturbine wheel comprises an insulating sleeve and an insulating disk, and

FIG. 5 shows a fifth embodiment of the rotor assembly in, wherein theshaft is positively installed in the turbine wheel in a positive manner.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

In the figures, the same or functionally equal components are providedwith the same reference numerals.

FIG. 1 shows a first embodiment of the rotor assembly 1 in alongitudinal sectional view. The rotor assembly 1 comprises a compressorwheel 2 for taking in and compressing combustion air, a turbine wheel 3for the expansion of exhaust gas and a shaft 4 with a rotational axis 5connecting the compressor wheel 2 with the turbine wheel 3 in arotationally fixed manner.

The rotor assembly 1 is provided especially for an exhaust gasturbocharger, which comprises a housing receiving the rotor assembly 1in a rotatable manner. The housing comprises an air guide section, anexhaust gas guide section and a bearing section. The compressor wheel 2is disposed in the air guide section, the turbine wheel 3 is disposed inthe exhaust guide section, and the shaft 4 is rotatably supported in thebearing section.

The exhaust gas turbocharger serves for increasing the performance of aninternal combustion engine. The internal combustion engine usually hasan combustion air intake duct, and an exhaust gas line, wherein the airguide section is arranged in the intake duct and the exhaust guidesection in the exhaust gas line. During the operation of the internalcombustion engine, the turbine wheel 3 is rotated by the exhaust gas ofthe internal combustion engine, and the compressor wheel 2 is rotated bymeans of the shaft 4, so that it takes in combustion air and compressesit.

The turbine wheel 3 is made of a metal aluminide or a high-temperatureresistant titanium aluminum alloy. The advantage of these materials is,in addition to a low heat expansion, is their favorable strength-densityratio, that is, they have a high strength with a low density. The massof the turbine wheel 3 is reduced by about half compared to a turbinewheel 3 consisting of for example the usual material Inconel 713 C. Themoment of inertia of the mass of the rotor assembly 1, whichcharacterizes the response behavior of the exhaust gas turbocharger, canthus be reduced considerably. The turbine wheel 3 as shown in FIG. 1 hasan axial opening 6 which extends fully through the turbine wheel 3 andin which the shaft 4 is received. Due to the opening 6, there areessentially no radial and centrifugal forces, which occur duringoperation of the exhaust gas turbocharger and which may lead, dependingon their size, to deformation and finally to breakage of the turbinewheel 3, effective between the shaft and the turbine wheel 3 via thesurface delimiting the opening 6 at its circumference.

If the turbine wheel 3 were made of a usual material (e.g. Inconel 713C), it would have a relatively low durability during operation with thearrangement according to the invention, as, due to the large mass thetensions occurring as a result of the centrifugal forces at the opening6 would be so large, that the strength of the usual material would betoo low for a continuous operation. The centrifugal forces need to beaccommodated in the radial direction and, at the same time, torsionalforces need to be transmitted in the tangential direction, which are sohigh that, with a turbine wheel 3 made of a usual material, theoperation of the exhaust gas turbocharger would result in early materialfailure of the rotor assembly 1.

The shaft 4 is accommodated in the opening 6, and the turbine wheel 3 isarranged adjacent an end head 7 of the shaft 4 and is connected to theshaft 4 for example by a press-fit. The press-fit already constitutes aform of the positive connection, as the connection between the shaft 4and the turbine wheel 3 is effected by means of frictional forces.

The compressor wheel 2 is positioned at an end 8 of the shaft 4 oppositethe first end head 7. A sleeve 10 is arranged on the shaft 4 between theturbine wheel 3 and the compressor wheel 2, which sleeve forms alow-friction bearing of the shaft 4 in the bearing section.

The shaft 4 comprises a tensioning element 9 arranged at the second end8 for the rotationally fixed connection of the turbine wheel 3 with thecompressor wheel 2, wherein an axial force transmission is provided bymeans of a tensioning element 9.

The shaft 4 includes the end head 7 in the shape of a nut, so that thefirst end 7 represents a tensioning engagement element which may also bein the form of a nut threaded onto the shaft 4. By the action of a forceprovided by the tensioning element 9, the rotationally fixed connectionbetween the turbine wheel 3, the shaft 4, the sleeve 10 and thecompressor wheel 2 can be established, whereby the connecting forcesextend mostly in axial direction of the shaft 4.

The sleeve 10 is in the form of a hollow cylinder and has areinforcement at its end facing the turbine wheel 3, in which an annularfirst recess 11 is arranged at the circumference of the sleeve 10. Therecess 11 serves especially for the reception of sealing elements.

In a second version of the exhaust gas turbocharger according to FIG. 2,the turbine wheel 3 includes an axial collar 13 extending from its endface 12, wherein the annular first recess 11 is arranged. The sleeve 10comprises a simple cylindrical structure without reinforcement.

It is an advantage of the second embodiment that the sleeve 10 has asmall wall thickness W, so that only a small expansion of the sleeve 10during the operation of the rotor assembly 1 as a result of heatgeneration can be expected during operation even at high rotationalspeed of the rotor assembly 1. Additionally, temperature stresses at thecollar 13 are smaller, so that an improved centering of the turbinewheel 3 on the shaft 4 is achieved.

In a further embodiment, an annular carrier 14 in the form of a ringcarrier with a U-profile is positioned in the first annular recess 11,the manufacture of which can be integrated into a manufacture processfor the turbine wheel 3, for example by a casting method. This ringcarrier 14 is provided for the reduction of wear and consists of acorresponding material, for example ceramics.

For ensuring the operation of the exhaust gas turbocharger, the rotorassembly 1 needs to be balanced as well as possible, which can beachieved by maintaining the radial position of the turbine wheel 3, theshaft 4, the compressor wheel 2 and the sleeve 10. The correspondingcentering may be obtained by a feature of a third embodiment as shown inFIG. 3. The turbine wheel 3 comprises a centering collar 15 at itscollar 13, by means of which the sleeve can be accurately fixed radiallywith respect to the turbine wheel 3. The sleeve 10 includes at its endfacing the centering collar 15 a recess 16, in which the centeringcollar 15 is accommodated.

As the bearing locations in the bearing section have to be kept as coolas possible, the heat transport from the turbine wheel 3 to the shaft 4or to the sleeve 10 has to be limited. For reducing the heat transportfrom the turbine wheel 3 to the sleeve 10, an air gap 19 is providedbetween a first surface 17 of the centering collar 15 facing away fromthe of the turbine wheel 3 and a second surface 18 of the second recessfacing the turbine wheel 3.

In a further embodiment, the end of the sleeve 10 facing the compressorwheel 2 is firmly connected to a bearing collar 25 of the compressorwheel 2.

In a fourth version according to FIG. 4, an insulating sleeve 20 in theshape of a hollow cylinder is arranged in, the opening 6 extending alongthe rotational axis 5 for the heat insulation and/or centering. Theinsulating sleeve 20 is connected to the turbine wheel 3 in rotationallyfixed manner by a press-fit. The shaft 4 is accommodated in theinsulating sleeve 20.

The arrangement of an annular insulating disk 21 between the collar 13and the sleeve 10 provides for further thermal decoupling of the hotturbine wheel 3 and the sleeve 10 particularly with a suitable choice ofmaterials. The heat transfer between the turbine wheel 3 and the bearinglocations in the bearing section to be kept cool can thereby be keptlow. As the rotor assembly 1 according to the invention is suitable fora ball or air suspension in the exhaust gas turbocharger, the cooling ofthe bearing locations is especially important with a ball or airsuspension bearings of the rotor assembly 1 due to very small bearinggaps.

In a fifth version according to FIG. 5, a positive connection isprovided between the turbine wheel 3 and the shaft 4. The shaft 4comprises a thread 22 at its first end 7, wherein the shaft 4 ispreferably a tension bolt. The opening 6 is formed only partiallyextending through the turbine wheel 3 starting from the collar 13 in thedirection of the rotational axis 5. A mating thread 24 for the positiveconnection of the turbine wheel 3 to the shaft 4 is provided at thethird end 23 of the opening 6 arranged opposite the collar 13.

With the firm axial engagement between the turbine wheel 3 and thecompressor wheel 2 via the sleeve 10 which also forms a bearingstructure for the rotor assembly 1 the high temperatures and the highcentrifugal forces to which the turbine wheel is subjected at highspeeds do not affect the engagement between the turbine wheel and thecompressor wheel via the sleeve 10.

1. A rotor assembly for an exhaust gas turbocharger, comprising a shaft (4) rotatable about an axis of rotation (5), a turbine wheel (3) mounted on the shaft (11) for the expansion of a first gaseous medium, and a compressor wheel (2) mounted on the shaft (4) for the compression of a second gaseous medium, and the turbine wheel (3) consisting of a metal aluminide, or of a high-temperature resistant titanium alloy and being rotationally fixed with respect to the compressor wheel (2) by means of the shaft (4), and the turbine wheel (3) having an axial opening (6) at least partially extending through the turbine wheel (3) and receiving the shaft (4) with a rotationally fixed connection between the turbine wheel (3) and the shaft (4) and a sleeve (10) disposed around the shaft (4) and arranged between the turbine wheel (3) and the compressor wheel (2) in firm axial engagement with the turbine wheel (3) and the compressor wheel (2).
 2. The rotor assembly according to claim 1, wherein the shaft (4) has a first end (7) firmly engaged with the turbine wheel (3) and a second end (8) provided with tensioning means (9) for axially engaging the compressor wheel (2) with the turbine wheel (3) via the intermediate sleeve (10).
 3. Rotor assembly according to claim 2, wherein the shaft (4) extends fully through the compressor wheel (3) and the tensioning means is a nut (9) threaded onto the shaft (4) for axially engaging the turbine wheel (3) and the compressor wheel (2) via the bearing sleeve (10).
 4. The rotor assembly according to claim 1, wherein the turbine wheel (3) comprises a collar (13) at an end face (12) of the wheel (12), provided with an annular first groove (11) for receiving sealing elements.
 5. The rotor assembly according to claim 4, wherein an annular carrier (14) is disposed in the annular groove (11).
 6. The rotor assembly according to claim 1, wherein a centering collar (15) is provided at an end face (12) of the turbine wheel (3) for engaging the sleeve (10).
 7. The rotor assembly according to claim 6, wherein the centering collar (15) and the sleeve (10) are firmly connected to each another.
 8. The rotor assembly according to claim 6, wherein an air gap is provided between a first surface (17) of the centering collar (15) facing away from the end face (12) of the turbine wheel (3) and a second surface (18) of the sleeve (10) facing the end face (12) of the turbine wheel (3).
 9. The rotor assembly according to claim 6, wherein an insulation sleeve (20) is arranged in the axial opening (6) between the turbine wheel (3) and the shaft (4).
 10. The rotor assembly according to claim 6, wherein an insulating disk (21) is provided between a collar (13) of the turbine wheel (3) and the sleeve (10).
 11. The rotor assembly according to claim 6, wherein the compressor wheel (2) is provided with a bearing collar (25) which is positively connected to an end of the sleeve (10) facing the bearing collar (25) for engagement with the compressor wheel (2). 